Please help! My jeep was running great before and now it feels under powered. My 1996 Jeep Cherokee Sport, 4.0 HO inline six, has just recently started shaking pretty bad when it is stopped at a light or idling. It ran like this for two days without the CEL coming on. NOW<The CEL was not on until yesterday and it says misfire in cylinder #3 and #4. I have replaced the IAC, TPS, CPS, Cap, Rotor, Pickup plate in bottom of Distributer, Plugs and Wires, along with new fuel filter and cleaned out Throttle Body. Still shakes and now the CEL blinks randomly. I was wondering if a catalytic converter could get plugged and cause this problem? No backfiring... Also it doesn't blow any smoke... Could it be a bad injector or cracked exhaust manifold? I also pulled both O2 sensors and with the rear one out it seemed to run better (not that much better though)... PLEASE PLEASE help! This is my DD and my pockets aren't very deep. Also I have checked engine mounts and they are good....

 

Two things you need to rule out are fuel injectors and an internal problem. Are you using any coolant?

1. Run a compression test on all cylinders and see where cylinders 3-4 fall. A compression gauge can be rented from most parts stores; call around The compression spec for the 4.0 is 120-150 psi with no more than a 30 psi variation between cylinders. Two adjacent cylinders with low compression is often a blown head gasket. If any cylinders are low with a dry compression test, perform a wet compression test which means you should introduce a small amount of oil into the cylinder and redo the test and see if the compression numbers come up. That wet test can tell you whether the problem is most likely piston rings or if the problem is in the head.

2. If compression numbers are in spec, try swapping one or both of the injectors from cylinders 3 & 4 to different cylinders and see if the misfire follows.

Good luck and keep us updated!

 

Thanks for the quick reply. My buddy has a garage and we were going to check the compression test tomorrow. I will try swapping the injectors tonight and see what it does... if it is the injectors, should I just replace them with stock injectors or try to upgrade from the single port injector? I forgot to mention that the Jeep has 181K on it and I have owned it for the last 43,000miles. I have changed the oil regularly and tried to baby this thing the best I could. I also have a new battery and new 2g wires... I'll post the results tomorrow night... thanks again.

 

Also I have not been using any coolant... Coolant levels seem to stay consistant and there is no sign of coolant mixing with the oil.

 

TJ is right, must confirm compression and rule out injector issues. Other possible causes are vacuum leak, and the TSB about carboned-up valves due to weak springs.

 

So here is where I am... I did a compression test and all the cylinders were fine, like I expected (I don't burn any oil or use any coolant, plus no smoke comes out of the exhaust). All were showing between 120-130psi. So then I was noticing that when I would pull the wiring off of the injector #3 and #4 (while the engine was running) it would not make a difference in how the engine would run. So I went ahead and REPLACED the injectors and all the O-rings and still no change. What the hell could it be? Please help if you might have any idea of what might fix this problem, or if anyone has encountered a problem like this... thanks for the any help.

 

When throttle is applied do they work and motor runs smooth or? I'd also bust out the test light

 

Do you have spark on those cyls? If you don't have spark, there will be no fire, and it won't matter if the injectors are plugged in or not.

If you have spark, I'd get ahold of a noid light, and see if those injectors getting a signal. If not, look at the wiring or ECM.

 

I have spark to all cylinders, good compression and signal to #1, #2, #5, #6 cylinders but when I disconnect the fuel injector wire, while the engine is running, to the #3 and #4;it doesn't change anything...

 

Belevedere is spot on with the next step here........

Noid light to verify injector pulse to the suspect cylinders. Noid light can be purchased inexpensively.

You must verify if the computer is sending a pulse to the fuel injectors. Or not, which is likely here. The driver electronics that sends pulses to the fuel injector is inside of the computer.

 

Inspect the wiring harness where it comes out of the fuel injector wiring manifold. There have been cases were the injector wires get chafed by the end of the fuel injector rail and/or the valve cover. You may have to cut the cable ties on the harness looms and move the looms so you can see under them.

 

So I went and bought a Noid Light Tester to test the injector wire to number #3 and#4 cylinder. I hooked the tester up to the #1 cylinder to see what the light would do when a cylinder works correctly, then I tested #3 and #4. When I hooked up #3 and #4 the light still shows a signal coming through. It still runs rough and the idle doesn't change when I disconnect #3 and #4 cylinder... if anyone has any ideas let me know. Should I try swapping out the PCM? What about "limp" mode?